Add simple build.hs script to extract the command list overview from the man pages
build.hs will:
* Extract the command name/short description from the main/git-* man pages.
* Replace the "Commands" section in the README file with a properly formatted
list of commands
Run manually:
$> runhaskell build.hs
This can be executed as part of a post-commit script to automatically update
the README file.